Today, Tumblr will now allow posting live streams from other apps (YouNow and others).
NOTE: Tumblr isn't launching its own live-stream tool – it's just allowing you to post and share streams from other apps.
Tumblr really, really wants you to notice these. Like, really.
You'll get a push notification when someone you follow goes live (egads?). Also, any live videos from people you follow will be pinned to the top of your dashboard for the duration of their stream.
People can also reblog your live streams to share with a larger audience.
There are more details on how to connect your Tumblr account to set up for streaming with these apps on the Tumblr support page.